Priority is claimed to Japanese application No. 2002-314148, filed Oct. 29, 2002, which is incorporated herein by reference.
1. Field of the Invention
This invention relates to a fuel cell system for use in fuel cell vehicles and the like.
2. Description of the Related Art
Because fuel cells which generate power by using hydrogen gas and an oxidizing gas as reaction gases produce water as well as power, an even larger consumption amount of hydrogen gas and oxidizing gas than is necessary for power generation is supplied in order to discharge this produced water from inside the fuel cell.
Accordingly, there is non-reacted hydrogen gas contained in the anode off gas discharged from the anode of the fuel cell (hereunder referred to as hydrogen off gas), and since this is emitted in this state, fuel consumption deteriorates. Therefore, in order to improve fuel consumption, a fuel cell system has been devised in which this hydrogen off gas is positively circulated using a device for raising the pressure, such as a hydrogen pump or the like, and mixed with fresh hydrogen gas and again supplied to the fuel cell, as shown, for example, Japanese Unexamined Patent Application, First Application No. 58-30075. Hereunder, a fuel cell system that circulates and reuses fuel in this way will be referred to as a circulation-type fuel cell system.
However, in the case of a solid polymer electrolyte membrane-type fuel cell, because the electrolytic conductivity is reduced and the power output decreases when the solid polymer electrolyte membrane dries out, it is necessary to maintain the solid polymer electrolyte membrane at a predetermined moistness in order to maintain a favorable power generating performance. Therefore, in solid polymer electrolyte membrane-type fuel cells, there are cases in which moistened hydrogen gas is supplied to the fuel cell, as shown, for example, Japanese Unexamined Patent Application, First Application No. 7-240220.
In the case of this fuel cell, the hydrogen off gas discharged from the fuel cell also contains moisture. Because of that, in the circulation-type fuel cell system provided with this solid polymer electrolyte membrane-type fuel cell, the hydrogen off gas containing moisture is circulated by the hydrogen pump or the like.
However, in the above circulation-type fuel cell system provided with this solid polymer electrolyte membrane-type fuel cell, there is concern that when the fuel cell system is stopped in a below-freezing environment, water content within the hydrogen pump freezes, so that it becomes difficult to start the hydrogen pump, and the discharge amount is reduced. When this happens, the amount of circulating hydrogen off gas is reduced when starting up at low temperatures, and as a result, there is concern that the amount of hydrogen supplied to the fuel cell is reduced, thus making stabilized startup of the fuel cell system difficult.
Furthermore, freezing of the hydrogen pump is not limited to the case where moistened hydrogen gas is supplied to the fuel cell, and may also occur in a circulation-type fuel cell system which does not actively moisten the hydrogen gas supplied to the fuel cell. That is because water is produced as well as power in fuel cells similar to the above, and this produced water is discharged together with the hydrogen off gas.
Also, in the case where the hydrogen pump is unable to drive for reasons other than freezing, and the discharge amount is reduced, the same difficulties arise.